1. Field of the Invention
The present invention relates to a semiconductor device including semiconductor elements such as diodes, transistors, and thyristors, and more particularly, to a composite semiconductor device having a function for overcurrent detection.
2. Description of the Related Art
As a high power semiconductor device in a composite semiconductor device having a function for overcurrent detection, an IGBT (Insulated Gate Bipolar Transistor) has been used. FIG. 5A is a perspective view (resin-sealed portion is omitted) showing an internal structure of the IGBT, FIG. 5B is a view showing an appearance of the IGBT, and FIG. 5C is a view showing an equivalent circuit, respectively. As shown in FIG. 5A, in the IGBT, an insulating substrate 1 is used, which includes conductive layers 2, 3, and 4 such as copper foils provided on a ceramic substrate. A semiconductor element chip (IGBT) 5 has a collector electrode (main electrode) on one major surface (lower surface) and an emitter electrode (main electrode) 6 and a gate electrode 7 on the other major surface (upper surface). The collector electrode of the semiconductor element 5 is mounted on the conductive layer 2 by soldering or the like, and the conductive layers 3 and 4 are electrically connected to the gate electrode 7 and the emitter electrode 6 through a metal wire or the like, respectively. The conductive layers 2, 3, and 4 are connected to external terminals 8, 9, and 10 of a collector (C), a gate (G), and an emitter (E), respectively. The semiconductor element described above is finally resin-molded and has an appearance shown in FIG. 5B.
In the case of the high power semiconductor device, for preventing the semiconductor device from unwanted breakdown caused by an overcurrent, a resistor 11 for detecting a current is connected in series with the main current path as shown in FIG. 6A, or a current transformer 12 is connected to the main current line as shown in FIG. 6B, thereby performing a current detection. Information for the overcurrent passage is then obtained from the current detection. The information is fed from T1 and T2 terminals or T3 and T4 terminals, and is fed back to a drive circuit of the semiconductor device to reduce the overcurrent.
However in the current detection using the resistor shown in FIG. 6A, a power loss occurs due to the product of the voltage developed across the resistor and the current flowing therethrough even in a steady energization state. Since the power loss is increased in accordance with an increase in steady current value, power efficiency is decreased in a semiconductor device using a high current. In addition, since a high allowable loss is required, the size of the resistor becomes large. Therefore, the current detection using the resistor is not generally employed in devices utilizing the high current.
Further, in the current detection using the current transformer shown in FIG. 6B, since the power loss does not occur unlike the method using the above resistor, a high current can be detected. However, in this method, a theoretically perfect DC current (di/dt=0) cannot be detected. Further, when a pulse current flows through the current transformer to detect the high current with a relatively long pulse width, a current transformer having a relatively large size must be used.
As described above, it is difficult to incorporate the current detector, such as current transformers, resistors, etc., in the semiconductor device. When the high power semiconductor device is practically used, a current transformer must be added to the high power semiconductor device in the case where the current is high, and a resistor must be also added to the high power semiconductor device in the case of a comparatively low current. At any rate, the handling of the resistor or current transformer is very complicated. The function for current detection has the above-described inconvenience.
A generally used composite semiconductor device having a function for overcurrent detection is shown in FIG. 7A.
The same reference numerals in FIG. 7A denote the same or corresponding parts as in FIGS. 5A to 5C. A semiconductor element 25 shown in FIG. 7A has a current detection electrode 26 which is connected to an external terminal 28 for current detection (Es) through both a metal wire and a conductive layer 27. In the composite semiconductor device, a current can be easily detected from the current detection external terminal. FIG. 7B shows an equivalent circuit of the composite semiconductor device.
In the semiconductor device described above, the detected output appears between the current detection external terminal 28 (or Es) and an emitter external terminal 10 (or E). Therefore, when a plurality of semiconductor elements connected in series with one another are used (usual inverters for three-phase motor drive or the like), electric potentials of ground terminals (emitter E) for current detection outputs in semiconductor elements are different from one another, so that the design of the feedback circuit is undesirably complicated. When the semiconductor device having the function for current detection described above is used, a semiconductor element must be newly designed and developed. In addition, in the developed semiconductor element, the area of the active region is decreased due to the electrode area provided for the current detection output. When equal rated current values are to be obtained, the chip size of the semiconductor element becomes large to increase the chip cost.
As described above, it is important to prevent the semiconductor device, particularly, the high power semiconductor device, from the breakdown caused by the overcurrent. For this reason, various overcurrent detecting means have been proposed. When the current detection resistor or the current transformer is connected in series with the main current path of the semiconductor element, it must be externally added to the semiconductor element since these parts are relatively large in size. The power loss occurs in the resistor, and the current transformer is not suitable for DC current detection. Therefore, these means are functionally inconvenient. In the semiconductor device having the function for current detection, the above problem can be improved. However, when a plurality of semiconductor elements are used in serial and parallel connections as employed in inverters for driving three-phase motors, common potential terminals cannot be often provided among detected signal output terminal pairs of respective elements, and the design of the feedback circuit is complicated. In addition, when a function for current detection is added to the semiconductor element, the chip size is undesirably increased.